I think improved diet has helped.
A year or so ago I noticed both my tangs had pin holes on their heads. It occurred to me that over the years I had become very lazy about what I fed them. Their diet had come to consist of caulerpa from the fuge and either raw shrimp or silversides, that's it. I just had been thinking because they were plump, that they were healthy.
I bought some selcon, added a wider variety of fish flesh to the diet, started feeding nori again, and added a fortified flake food. I also fed them broccolli for a while after reading it is a good source of Vit C and that helps with HLLE. I don't know how much the broccolli helped, but I was feeling so guilty for letting them get this way that I was trying anything (and my sailfin loves it) They healed up. Whew!!!